Pseudomonas exotoxin A (PE) is a bacterial toxin with cytotoxic activity that may be effective for destroying or inhibiting the growth of undesirable cells, e.g., cancer cells. Accordingly, PE may be useful for treating or preventing diseases such as, e.g., cancer. However, PE may be highly immunogenic. Accordingly, PE administration may stimulate an anti-PE immune response including, for example, the production of anti-PE antibodies and/or T-cells, that undesirably neutralizes the cytotoxic activity of PE. Such immunogenicity may reduce the amount of PE that can be given to the patient which may, in turn, reduce the effectiveness of the PE for treating the disease, e.g., cancer. Thus, there is a need for improved PE.
Several deimmunized Pseudomonas exotoxins (PE) are known in art. The domain II deleted versions (for example, PE24) may be less immunogenic and may cause fewer side effects (such as, for example, capillary leak syndrome and hepatotoxicity) as compared to PE38, which contains domain II. Without being bound to a particular theory, it is believed that the reduced immunogenicity and fewer side effects of PE24 could, at least in part, be due to the reduced size of PE24, which disadvantageously results in a shorter serum half life. Different furin cleavable linkers may be employed in PE24 variants. PE immunoconjugates have mostly used dsFv fragments as targeting moieties. Such deimmunized Pseudomonas exotoxins (PE) are described in, for example, International Patent Application Publications WO2005052006, WO2007016150, WO2007014743, WO2007031741, WO200932954, WO201132022, WO2012/154530, and WO 2012/170617.
Previous immunotoxins have many disadvantages. For example, deimmunization of previous immunotoxins has been incomplete with respect to the human B-cell epitopes because immunogenic reactions still occurred. In addition, the deimmunization of previous immunotoxins was accompanied by a reduced cytotoxic potency. For example, a LO10 deimmunized PE variant described in WO 2012/170617 provided a loss of potency of at least 40% compared to wild type (WT) PE and other PE variants. In International Patent Application Publication WO2013/040141, Pseudomonas exotoxins with less immunogenic B-cell epitopes have been described. In the PE variant LRO10, all B-cell epitopes were removed. This, however, also led to a reduction of cytotoxicity towards tumor cells.
In addition, fusion of a dsFv with domain II deleted versions of PE (PE24) have a shorter serum half life due to their reduced overall size as compared to dsFv fusions with PE38. The linkers of previous immunoxins also contained T-cell epitopes and poor developability such as, for example, a poor stability at 37° C. In addition, previous anti-mesothelin (MSLN) immunotoxins have only used mouse-derived dsFv fragments fused to PE, which may further contribute to immunogenicity. International Patent Application Publication WO 2012/154530 refers to Pseudomonas exotoxin variant chimeric molecules with short flexible linkers which improve the cytotoxicity towards tumor cells.